Cation-pi interactions: Difference between revisions

Cationic moieties that are within 6.0 Å of the face of an aromatic ring (phenylalanine, tyrosine, or tryptophan) may engage in polar interactions called cation-pi interactions (cation-π interactions)<ref name="dad2013">PMID: 23214924</ref>.

The flat face of an aromatic ring has a partial negative charge owing to the delocalized [[#Chemistry|pi electrons]]. <!-- ([http://www.molphys.leidenuniv.nl/monos/smo/index.html?basics/photophysics.htm schematic], [http://www.webelements.com/shop/product.php/129/molyorbital_molecular_orbital_organic_structures_set physical model]). -->

Cations such as the sidechains of Lys or Arg, cationic ligands, or metal cations often align themselves centered over the faces of aromatic rings. Over one fourth of Trp's in the Protein Data Bank interact with cations, and 99% of significant cation-pi interactions occur within a distance of 6.0 Angstroms <ref name='GD'>PMID: 10449714</ref>. Cation-pi interactions make a significant contribution to the overall stability of most proteins. Gallivan and Dougherty (1999)<ref name='GD' /> concluded that "cation-pi interactions should be considered alongside the more conventional hydrogen bonds, salt bridges, and hydrophobic effects in any analysis of protein structure". They can also contribute significantly to intermolecular contacts and interactions with ligands.

 

*[http://firstglance.jmol.org FirstGlance in Jmol] finds and displays potential cation-pi interactions, as described [http://firstglance.jmol.org/notes.htm?#catpi here].

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*[http://proteinexplorer.org Protein Explorer] (requires CHIME plugin) finds and displays potential cation-pi interactions, as described [http://proteinexplorer.org/cationpi.htm here]. -->

 

Six-carbon aromatic rings occur in the sidechains of 3 of the [[amino acids|20 standard amino acids]]: namely [[Amino_Acids#Phenylalanine|phenylalanine]], [[Amino_Acids#Tryptophan|tryptophan]], and [[Amino_Acids#Tyrosine|tyrosine]]. One [http://en.wikipedia.org/wiki/P-orbital p atomic orbital] on each aromatic carbon overlaps in a pi ("sideways") fashion with its two neighbors to form a [http://en.wikipedia.org/wiki/Conjugated_system conjugated] pi (&pi;) orbital system.

The pi electrons of aromatic rings comprise [http://en.wikipedia.org/wiki/Delocalized_electron delocalized] annular clouds above and below the ring plane ('''see Figure at right'''). Proximity of a positive charge to one face of the ring attacts and polarizes the pi electron cloud. In the proper conformation, this proximity forms an energetically significant '''cation-pi''' interaction (see '''Figure above''')<ref name="lillya">[[User:Eric Martz|Eric Martz]] thanks [http://www.cns.umass.edu/faculty/directory/c-peter-lillya Peter Lillya] for help writing this paragraph.</ref> .

<StructureSection load='1gai' size='350' side='right' scene='Cation-pi_interactions/Nicotine_catpi/5' caption='Cation-pi interactions'>

The nicotinic acetylcholine receptor is a ligand-gated ion channel that responds to the neurotransmitter acetylcholine. It occurs in the central and peripheral nervous systems, and in the neuromuscular junction that controls muscle contraction. It has multiple forms, notably a ''neuronal type'' found in the brain, and a ''muscle type'' found in neuromuscular junctions. The nicotinic acetylcholine receptor is the basis for nicotine addiction, which kills over 4,000,000 people annually<ref name="XD">PMID: 19252481</ref>. As stated by Xiu ''et al.''<ref name="XD" />:

 

 

 

In 1998, Zhong ''et al.'' (with Dougherty)<ref name="zhong">PMID: 9770444</ref> provided evidence that the [http://en.wikipedia.org/wiki/Nicotinic_acetylcholine_receptor cation in '''acetylcholine'''] engages in a cation-pi interaction with Trp149 in the '''neuronal-type''' receptor, making an important contribution to the binding affinity. In 2002, Beene ''et al.'' (with Dougherty)<ref name="beene">PMID: 12162741</ref> provided evidence that no such cation-pi interaction is involved when '''nicotine''' binds to the '''muscle-type''' receptor, thus accounting in part for the lower affinity. In 2009, Xiu ''et al.'' (with Dougherty)<ref name="XD" /> provided evidence that a strong cation-pi interaction occurs between the cation of '''nicotine''' and Trp149 in the '''neuronal-type''' receptor. The lower affinity of nicotine for the muscle-type, compared to the neuronal type receptors, is accounted for by differences in cation-pi interaction strength and the absence of a nicotine-receptor hydrogen bond in the former<ref name="XD" />.

 

The crystal structure of a molluscan acetylcholine-binding protein ([[1uw6]]) confirms the existence of a cation-pi interaction between the homologous Trp (Trp143) and the nicotine cation (<scene name='Cation-pi_interactions/Nicotine_catpi/5'>restore initial scene</scene>). Also seen is the <scene name='Cation-pi_interactions/Nicotine_catpi/6'>hydrogen bond</scene> proposed between nicotine and the main-chain oxygen of the proximal Trp for the rat receptor<ref name="XD" />. The molluscan crystal structure also reveals a remarkable <scene name='Cation-pi_interactions/Nicotine_catpi/7'>''aromatic box'' of 3 Tyr and 2 Trp surrounding nicotine</scene>.

 

 

*[[1gai]]: a 472-amino acid chain (glucoamylase) with a <scene name='Cation-pi_interactions/1gai_pi4/1'>spectacular cluster of four aromatic rings (two Trp's, two Tyr's) around a single Lys108</scene>.